Presidential Green Chemistry Challenge: 2012 Greener Reaction Conditions Award
Cytec Industries Inc. (Acquired by Solvay)
MAX HT® Bayer Sodalite Scale Inhibitor
Summary of Technology: The Bayer process converts bauxite ore to alumina, the primary raw material for aluminum. The process involves extracting alumina trihydrate from bauxite ore using hot caustic solution. After separating out the insoluble solids, the alumina trihydrate is precipitated and the spent liquor is recycled. Heat exchangers re-concentrate the liquor to the optimum concentration of caustic and then heat it to the proper temperature for digestion. Silica present as silicates, primarily clay materials, dissolves quickly in typical Bayer liquor used to digest alumina, resulting in the liquor being supersaturated in silica, particularly after precipitation of the alumina trihydrate. The silica in the liquor reacts with the caustic and alumina on the hot surfaces of the heat exchangers; as a result, sodalite scale (i.e., crystalline aluminosilicate) builds up on the heat exchangers and interstage piping in the process. This reduces the efficiency of the heat exchangers. Periodically, Bayer process plant operators must take the equipment off line for cleaning that involves removing the scale with sulfuric acid. The used acid is a waste stream that requires disposal. In addition to the acid cleaning, much of the interstage piping requires cleaning with mechanical means such as jackhammers to remove the scale.
Cytec developed its MAX HT® Bayer Sodalite Scale Inhibitor products for the Bayer process. There are no other scale inhibitors on the market for this application. The active polymeric ingredient contains silane functional groups that inhibit crystal growth by incorporation into the crystal or adsorption onto its surface. The polymers have molecular weights in the range 10,000 and 30,000. Their synthesis involves polymerizing a monomer containing a silane group or reacting polymer backbone with a reagent containing the silane group. Dosages range from 20 to 40 ppm. Assessments of these polymers under EPA's Sustainable Futures Program indicate low overall concern for human health and the aquatic environment.
Eliminating sodalite scale from heater surfaces has many benefits. Heat recovery from the steam produced in various unit operations is more efficient. Increased evaporation makes the countercurrent washing circuit more efficient and reduces caustic losses. Reducing the use of steam reduces emissions from burning carbon-based fuels. Finally, reducing the sulfuric acid used to clean heaters reduces both worker exposure and waste. Typically, MAX HT® inhibitor increases the on-stream time for a heater from 8–10 days to 45–60 days for digestion and from 20–30 days to over 150 days for evaporators.
There are about 73 operating Bayer process plants worldwide with annual capacities of 0.2–6 million tons of alumina per plant; most plants are in the 1.5–3 million ton range. Eighteen Bayer process plants worldwide have adopted this technology; seven more plants are testing it. Each plant using MAX HT® saves $2 million to $20 million annually. The realized annual energy savings for all plants together are 9.5 trillion to 47.5 trillion Btu, which is the equivalent of about 1.1 billion to 7.7 billion pounds of carbon dioxide (CO2) not released to the atmosphere. Fewer cleaning cycles and less acid per cycle result in a realized annual hazardous waste reduction of 76 million to 230 million pounds for all plants together.
Podcast on the technology:(MP3, .98 MB, 1:04 seconds)
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